The invention relates to a soundboard of composite fibre material construction for use for an acoustic musical instrument, particularly a bowed stringed instrument.
However, the invention can also be used advantageously for other acoustic musical instruments (such as guitars and pianos) which are provided with a resonant body or resonant back-plate.
In recent years attempts have been made to produce the soundboards of acoustic musical instruments in composite fibre material construction. Structures of composite fibre material construction generally consist of long fibres which are preferably oriented in certain directions and a carrier or matrix material which is generally a thermosetting or thermoplastic plastics material. In the preferred embodiment of the invention this is an epoxy resin system.
The previous efforts to produce soundboards of composite fibre material construction intended for acoustic musical instruments are aimed without exception at copying as well as possible the acoustic characteristics of the wood which is to be substituted. Examples of these attempts in the previously known prior art are provided for instance by DE 37 38 459 A1, EP 0 433 430 B1, U.S. Pat. Nos. 5,895,872 and 5,905,219. Thus DE 37 38 459 A1 aims at xe2x80x9ca macroscopic heterogeneity almost equal to the woodxe2x80x9d and states as the object that xe2x80x9cthe composite materialxe2x80x9d should xe2x80x9chave similar characteristics to sprucexe2x80x9d.
An unsatisfactory feature of these previously known soundboards of composite fibre material construction appears to be that from the acoustic point of view they are equivalent but in no way superior to very good solid wood soundboards of traditional construction.
The object of the invention, therefore, is to create a soundboard of composite fibre material construction which has a perceptibly better acoustic quality by comparison with excellent soundboards of traditional construction. In particular the soundboard according to the invention should have substantially higher radiated power whilst retaining the usual and desirable timbre of a solid wood soundboard.
This object is achieved according to the invention by the provision of a soundboard formed by a fibre coating of single-layer and at the same time multidirectional construction.
In detail, the invention is based on the following considerations and tests:
The cause of the sound radiation of the instrument is its characteristic vibrations. The frequencies and mode shapes of the eigenmodes of vibration crucially determine the timbre of the instrument. The formation of the eigenmodes of vibrations is again dependent upon certain material properties, amongst which the anisotropy of the wood is of outstanding importance. Anisotropy is understood to mean the directionality depending upon the physical properties of a material. The anisotropy of the velocity of sound of the longitudinal waves, i.e. the ratio of velocity of sound in the longitudinal direction to velocity of sound in the cross direction of the run of the fibres, is approximately 4:1 in the case of spruce wood and is thus very pronounced. The velocity of sound in the fibre direction which is approximately four times as great as the velocity of sound across the fibre may be attributed to the higher longitudinal bending strength of the spruce wood. The high stiffness in the longitudinal direction of the fibre also appears sensible because of the great forces occurring in this direction (because of the string tension).
Furthermore, in the conventional bowed stringed instrument there is a very good conformity between the anisotropy of the velocity of sound and the outline proportions (length to width) dictated by playing techniques, which are likewise of the order of magnitude of 4:1.
For these reasons it is necessary for the anisotropy of the soundboard produced from composite fibre material to correspond to the anisotropy of the conventional soundboard produced from solid wood. Otherwise the requirement to retain characteristic frequencies and characteristic vibrational shapes (and thus the desired and required timbre) is not met.
It might be thought that the required anisotropy could be produced by positioning various unidirectional layered fibre structures at certain angles crosswise one above the other and applying them to both sides of the core plate. The usual procedure is to build up a composite fibre structure in this way from stacked laminate layers when the object is to adapt the physical properties of the structure to the loading directions of the component. The angles which the longitudinal directions of the fibres of the various unidirectional layers of fibres assume with respect to one another then determine the ration of longitudinal stiffness to cross stiffness [see: Michaeli/Huybrechts/Wegener: xe2x80x9cDimensionieren mit Faserverbund-kunststoffenxe2x80x9d, Munich, Vienna 1994, page 61]. The previous attempts at producing soundboards from composite fibre materials follow this usual procedure. They are always built up from a more or less great number of different layered fibre structures or fibre meshes laid one above the other (laminates). cf. for instance DE 3737459, DE 69023318 T2; U.S. Pat. Nos. 5,955,688 or 6,087,568. All of these attempts rightly take account of the fact that the use of one single unidirectional layer of fibres on each side of the core plate is not as a rule sufficient to produce the required anisotropy. On the other hand, these conventional approaches to a solution underestimate an acoustically essential property of soundboards:
The vibration levels of the characteristic vibrations are crucial for the sound radiation of the instrument. They are dependent upon the vibrating mass of the soundboard, the acoustic significance of which results from the following correlation: The vibration resistance (so-called impedance) which the soundboard opposes to the exciting alternating force generated by the string vibrations is greater the higher the vibrating mass of the soundboard is. In order to achieve high vibrating speeds (so-called velocity) of the soundboard and thus the most effective possible sound radiation of the instrument, with a given excitation force the lowest possible vibration resistance and thus the lowest possible vibrating mass are necessary.
Since in the case of composite fibre sandwich constructions the predominant proportion of the total mass is provided not by the core plate but by the fibre coating, the total mass depends above all on the number of composite fibre coatings which is necessary.
This is apparentxe2x80x94by way of example for a violinxe2x80x94from the following numerical example: The average total mass of a conventional violin top plate made from spruce wood is between 60 and 75 grams. Soundboards having the same geometry and made from composite fibre material provide the following total masses, depending upon the number of fibre coatings applied (in the case of fibre coatings with a weight per unit area of 100 g/m2):
With one fibre coating in each case on the upper and lower face of the core plate: 46 grams total mass of the soundboard.
With two fibre coatings in each case on the upper and lower face of the core plate: 68 grams total mass of the soundboard.
With three fibre coatings in each case on the upper and lower face of the core plate: 91 grams total mass of the soundboard.
Thus it is clear that already with the use of only two unidirectional fibre coatings per face of the core plate, and thus the minimum number of fibre coatings necessary in order to produce the anisotropy, there are no longer any acoustic advantages over the conventional spruce soundboard.
With these considerations as a starting point, therefore, the invention follows a fundamentally different route in order to the anisotropy of the soundboard of composite fibre material construction in the required manner.
Whereas in the previous attempts at solutions for producing soundboards as a composite fibre sandwich the core plate is completely coated with a more or less large number of layers of fibres lying crosswise one above the other, in the solution according to the invention the multidirectional fibre alignment is achieved by means of a single-layer fibre coating or only part-zones of the core plate are provided with a fibre coating. Depending upon the fibre layer pattern the individual zones of the plate acquire different stiffness ratios between the longitudinal stiffness and the cross stiffness due to the degree and frequency of the changes in fibre direction.
The requirement for a single-layer and at the same time multidirectional fibre coating defines a layered fibre structure which in one single layer changes its fibre direction. In this case the fibres of individual fibre groups extend in the same direction, that is to say they are oriented as if xe2x80x9ccombedxe2x80x9d. Thus this is not a tangled fibre layer in which the fibres are likewise disposed multidirectionally; whereas in the tangled fibre coating the individual fibres are xe2x80x9cmixed up togetherxe2x80x9d, that is to say disposed randomly, in the fibre coating according to the invention due to the xe2x80x9ccombedxe2x80x9d arrangement as fibre groups the individual fibres form common linear fibre patterns. In contrast to the tangled fibre coating in which the individual fibres overlap at any angles, because of the xe2x80x9ccombedxe2x80x9d fibre orientation in the fibre coating according to the invention possible overlaps predominantly have small angles between individual fibres.
The term xe2x80x9csingle-layerxe2x80x9d does not exclude the possibility that individual fibres can be superimposed on one another to a certain extent because of their small cross-section within the matrix system in which they are embedded. Such superimposition of fibres of a single-layer fibre coating cannot be avoided as a rule using manufacturing techniquesxe2x80x94even when using prepregsxe2x80x94since during the liquefaction phase of the matrix system up to its ultimate hardening the fibres have a certain freedom of movement. Rather, the term xe2x80x9csingle-layerxe2x80x9d provides a definition which excludes the provision of a multi-layer construction such as is given in the conventional crosswise and/or layered construction by a plurality of fibre coatings or fibre meshes lying one above the other.
The reduced number of fibre layers according to the invention (with simultaneous production of the required anisotropy due to the changes in direction of the fibre coating) permits the production of substantially lighter soundboards by contrast with the prior art. Since, as explained, the vibrating mass of the soundboard is inversely proportional to the achievable speed of vibration (velocity), the solution according to the invention provides a higher sound radiation by contrast with the previous soundboards of composite fibre material construction and by contrast with the conventional solid wood soundboards with the same anisotropy and thus the same timbre.
Thus the soundboard according to the invention enables instruments to be built which correspond to the conventional instruments made from solid wood as regards the hearing habits (sensing the timbre) but which are markedly superior to the traditional instruments as regards their acoustic efficiency.